The invention generally relates to the field of imaging printing plates on a platesetter or imaging film on an imagesetter.
The process of transferring text and/or graphic information from electronic form to visual form on an imagable medium is called imaging. The information can be transferred to an imagable media using light such as produced by a laser beam or beams. The imagable media may be a printing plate or film that is sensitive to the wavelength of the laser beam(s) used to accomplish imaging.
The laser beam used to image a printing plate or film is often modulated with pulses. The laser beam must be calibrated to assure proper exposure of the imagable media is achieved.
A laser beam used to image a printing plate and/or film media is focused to a sharp point. This focal point is aligned on or with the surface of the media to be imaged. If the focal point of the laser beam falls short of the media surface, or is located beyond the media surface, the resulting image placed on the media is distorted. Consequently, proper positioning of the focal point of the laser beam or beams is crucial to producing a distortion free image on the media.
Preparation for imaging a printing plate or film includes calibration of the laser beam or beams. Various parameters of a laser beam (hereafter laser beam means a single beam OR may mean a plurality of laser beams) must be measured at precisely the focal point of the laser beam. This is often accomplished with a photo-detector or other light sensor. Often a single laser source or a plurality of laser sources are configured to provide a single line of laser light that is coupled to a modulating device such as a grating light valve (GLV). The GLV effectively creates a large number of individual light beams from the single laser light line. This is accomplished by manipulating small components inside the GLV so each individual light beam is turned on, and then off resulting in a pulse of light. Consequently, the original single line of laser light supplied to the GLV is transformed into a plurality of light pulses forming a train of pulses.
Characteristics of each pulse must be measured. Since each pulse must be measured, a slit sensor can be employed to separate or isolate a single pulse from the rest of the pulses. The slit sensor must be placed at precisely the focal point of the laser beam so as to measure the characteristics of the portion of the laser beam that actually strikes the surface of the imageble media. The slit sensor reflects all other pulses except a single pulse. This reflected light energy must not be directed back toward the laser source (or GLV) producing the pulsed laser beam as the laser source or GLV may be damaged. Tilting the slit sensor results in an offset between the focal point of the laser beam and the actual slit introducing an error in the measurement. The problem is exacerbated by vertical movement of the laser beam as various components expand or contract due to temperature fluctuations.
What is needed is a device that allows a single pulse to be isolated from a plurality of pulses at the focal plane of a laser beam without damaging the laser source or GLV. Further, the device must compensate for a vertically drifting laser beam so the focal point of the laser beam remains precisely aligned on the portion of the device that separates the pulses.
An object of the invention herein is to maintain a position of the focal point of a laser beam onto a slit aperture as said laser beams drifts vertically.
A further object of the invention herein is to attenuate the amplitude of the laser beam before said laser beam illuminates said slit aperture.
Another object of the invention herein is to redirect light away from a laser beam source that is reflected by a surface of the invention.
An object of the invention herein is to separate a single pulse of light from a plurality of light pulses modulated onto said laser beam.